1. Field of the Invention
This invention relates to an ink jet spray head in which the ink spray is controlled by modulation of an air stream. More particularly it relates to such a spray head that forms part of a system for reproducing and enlarging color images under computer control.
2. Description of Related Art
U.S. Pat. Nos. 474,158; 653,496; 649,431; 1,272,752; 2,264,564; 1,638,550; 1,703,359; 1,299,290; 2,550,404; 2,721,763; 4,020,990; 4,079,893; 4,102,500; 4,161,289; 4,171,097; and 4,606,501 are typical of ink jet spray heads in which a flow of air across an ink meniscus atomizes and sprays ink upon the medium. The flow of ink is controlled by varying the quantity of air admitted to the spray head or by the position of a needle extending into the spray orifice or by other valve arrangements. U.S. Pat. No. 767,510describes a similar spray head in which the control needle is stationary and the tube forming the orifice around the needle is moved to adjust the spray intensity. U.S. Pat. Nos. 1,179,095; 1,294,190; 1,333,488 describe spray heads in which the ink is fed by gravity or other independent pressure means into the spray orifice and compressed air is provided only to atomize the ink, not to withdraw it from the ink reservoir.
The foregoing patents describe systems in which, for the most part, the spray heads are hand operated. Various systems have been developed for automatically spraying enlarged color prints, such as for billboards, in which an original image is scanned to produce signals that operate the reproduction equipment. U.S. Pat. No. 1,709,926 describes a system in which three separate ink jets are controlled by data from an original image while the heads scan the medium being sprayed. U.S. Pat. No. 3,553,371 describes a multi-color imaging system in which the ink flow rate is controlled by changing the pressure of the air producing the atomization. The same patent also describes a spray head in which the ink flow is controlled by a mechanical flow valve operated from the input data signals. U.S. Pat. Nos. 4,914,522 and 4,999,651 describe spray heads in which the flow of ink is controlled by pulse width modulation of air supplied to the spray head.
U.S. Pat. No. 4,403,234 to Miura et al. and U.S. Pat. No. 5,119,110 to Aruchi et al. disclose spray heads for depositing droplets of ink, but neither patent discloses any means for producing a transverse asymmetrical relationship between the flow of the air and the ink in the area of the exposed end of the spray nozzle.